In recent years, as a printing plate for off-set printing, a large number of computer to plate (CTP) systems have been developed and put into practical use, which imagewise exposes directly to a planographic printing plate material employing laser light based on digital image data to record an image on the material. As the CTP systems, there are a silver salt system, a radical polymerization system and a thermal system, which differ in characteristics.
The radical polymerization system has characteristics providing high sensitivity and high printing durability, and is widely applied in printing fields such newspaper printing. The general radical polymerization is disclosed in many literatures such as Japanese Patent O.P.I. Publication No. 10-104835. A composition comprising an arene compound as a polymerization initiator is disclosed in Japanese Patent O.P.I. Publication No. 1-152109. Japanese Patent Publication No. 2755723 discloses, as a composition providing high sensitivity and high printing durability, a photopolymerizable composition which comprises a polymer binder, a free-radical polymerizable compound, a photoreduction dye (photo-initiator), and a trihalomethyl compound and a metallocene compound as another photo-initiator, and discloses in the examples a composition comprising an alcohol soluble eosin, a trihalomethyltriazine, and a titanocene compound. A composition disclosed in this patent document provides excellent sensitivity and printing durability, which is preferred, however, a planographic printing plate material comprising the composition is greatly lowered in storage stability after long term storage, resulting in insufficient practical properties.
The radical polymerization system generally employs a process comprising exposure, heat treatment, water washing, alkali development, and water washing in that order. This system is a preferred one to obtain high printing durability by completing the radical polymerization. However, when printing plate materials having a different thickness were processed, the printing plate materials are heated at preheating to different temperature due to the different thickness, producing problem of causing development failure. As the countermeasure, an attempt has been made in which arrangement of rollers in an automatic processing machine (automatic processor) or pre-determination (provision) of preheating conditions to each plate thickness is examined.
As an effective method to widen the latitude as printing plate materials to various preheating conditions, there is a method which increases solubility of light sensitive layer. Typically, a resin having a high acid value or a photoinitiator system for accelerating reaction by heating has been studied. However, satisfactory preheating latitude has not yet obtained considering printing durability. Further, the above negative-working photopolymerization type printing plate material has problem in that sludge is likely to be produced in a developer, and does not have a satisfactory running property.